This simplifies the implementation of the <markdown> component slightly,
and makes its inputs more consistent with other recently written components.

 - Replace use of 'ngModel' with 'text' input property and 'onEditText'
   output event.

 - Add tests for preview state.

 - Add tests for rendering and editing annotations that have no text.

 - Optimize editor initialization by not creating the toolbar's DOM
   elements until the user starts editing the annotation, via using
   ng-if rather than ng-show to hide the toolbar in view mode.